Preheat your oven to 325°F (163°C).
In a medium bowl, whisk together 2 ½ cups (300 g) all-purpose flour, 2 teaspoons cinnamon, ¼ teaspoon nutmeg, 1 teaspoon salt, and 1 ½ tablespoons baking powder. Set aside.
In a large mixing bowl, whisk together 1 ½ cups granulated sugar, ½ cup brown sugar, ¾ cup melted butter, and ¼ cup avocado oil until smooth and glossy. Add 3 large eggs, one at a time, whisking after each addition. Stir in 1 teaspoon vanilla extract until fully combined.
Mix in 1 cup unsweetened applesauce and gently fold in ½ cup active sourdough starter (bubbly and recently fed). Don’t overmix—this helps the cake stay tender.
Add about one-third of the dry ingredients to the wet mixture and stir gently. Pour in half of the ¾ cup apple cider and stir again. Repeat this process, alternating dry ingredients and cider, ending with the last of the dry mixture. Stir only until combined and no streaks of flour remain. The batter should be thick but pourable.
Generously grease a Bundt pan with butter, making sure to coat every ridge and curve. Dust with flour or sugar—sugar adds a light caramelized crust and helps the cake release easily.
Pour the batter evenly into the prepared Bundt pan and smooth the top with a spatula. Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. Start checking around the 50-minute mark.
Let the cake cool in the pan for 15 minutes, then gently loosen the edges with a butter knife. Carefully invert the cake onto a wire rack and allow it to cool for at least 1 hour before adding the coating.
In a small bowl, combine ½ cup granulated sugar and 2 teaspoons cinnamon. Brush the warm cake with 2 tablespoons melted butter, then sprinkle or roll the cake in the cinnamon sugar mixture, pressing gently so it sticks.
Slice and serve warm or at room temperature. The flavor deepens and the texture becomes even softer the next day—perfect with a cup of coffee or warm apple cider.